Liquid crystal display devices are now well known and widely used in a variety of applications. One of the principal reasons for their popularity is their small size and low power consumption. In particular, liquid crystal cells of the host-guest variety have become increasingly popular because they do not require polarizers; they have color capability; and they are intrinsically brighter than other types of liquid crystal cells.
One of the most significant forms of guest-host liquid crystal cells is one in which a pleochroic dye is dissolved in the liquid crystal host. The host liquid crystal, in the preferred form, contains an optically active (chiral) material which causes the liquid crystal molecules to exhibit a helical order with their optical or nematic director parallel to the cell substrate. The optically active material is preferably a cholesteric compound although other materials may be used. Pleochroic dyes are characterized by the fact that they exhibit anisotropic optical behavior; i.e., light is absorbed along one axis of the dye molecule (the long axis) and is transmitted along the other axis. The guest dye molecules dissolved in the liquid crystal host spontaneously align themselves with the helically ordered liquid crystal molecule so that their long, or light-absorbing axes are also parallel to the cell substrate.
In the absence of an electric field, the dye molecules are aligned in a helical order with their long axes parallel to the cell electrodes, and absorb light passing through the cell. When an electric field is applied to the cell, the helix unwinds and the host liquid crystal molecules assume a homeotropic order, i.e., at right angles to the cell electrodes. The guest dye molecules align themselves similarly. As a result, the long axes of the dye molecules also assume a homeotropic order and are oriented to transmit light through the cell.
The effectiveness of the guest dye molecules in absorbing light depends on the degree to which they are aligned with the host liquid crystal molecules. Unless there is a high degree of alignment, (i.e., a high order parameter) some light will be transmitted even though the cell is in the light absorbing or non-transmissive state. In addition, light is elliptically polarized in passing through a liquid crystal host with helical molecular ordering so that a fraction of the light is transmitted even in the absence of an electric field. For these reasons, imperfect ordering and elliptical light polarization, some light is always transmitted through the background portions even though the host liquid crystal and the guest dye in these background portions are helically ordered. The principal result is that the contrast ratio between the indicia and background portions of the cell is not high; with contrast ratios in the order of 4:1 to 10:1 being typical.
A number of solutions have been proposed for making the background portion darker and increasing the contrast ratio. One such solution is to increase cell spacing. However, when the cell spacing is increased, cell brightness is reduced and the cell driver voltage required to unwind the helix is increased, thus making this a far from ideal solution. It has also been suggested that background absorption can be increased and contrast ratio improved by increasing the dye concentration in the liquid crystal host. However, because of the relatively low solubility of dichroic dyes in the liquid crystal host, and the reduced brightness, this solution is also of limited usefulness.
Applicant has found that the contrast ratio of a back-lighted transflective liquid crystal display can be enhanced by orders of magnitude to produce contrast ratios in the range of 500-600:1.
The improvement in contrast ratio is brought about by ensuring that in the transmissive mode light from the rear of the cell only passes through the areas of the indicia-forming electrodes while virtually completely blocking illumination of the background portions of the cell. This is achieved by providing a light mask between the light source and transflector at the rear of the cell. The mask has light transmitting portions which are spatially aligned with, and have the same shape as, the indicia-forming cell electrodes. The remaining portion of the mask is opaque and essentially blocks all light from the background portion of the cell. In this fashion, the contrast ratio of the display is enhanced and the illuminated display indicia are much more readily visible than is the case without the use of the contrast enhancing light masking means.
The term "transflective" liquid crystal device is used broadly to describe an arrangement in which a liquid crystal cell may be operated in a reflective mode, a transmissive mode, or simultaneously in both modes. In the reflective mode light, whether ambient or from a specific source, enters the front of the cell and passes through the liquid host-guest solution to a reflector contained either within or in back of the cell. The light striking the reflector passes back through the solution to the front plate. In the transmissive mode, light from a source positioned in the back of the cell illuminates the rear of a transflective element which reflects most of the light striking its front face while passing a portion of any light illuminating its rear. In the transmissive mode, light illuminating the rear passes directly through the liquid crystal host-guest solution to the front of the cell. Transflective LCD's are sometimes used as instrument displays in an aircraft cockpit. During daylight, when the ambient lighting level is high, the LCD is operated in the reflective mode only by de-energizing the back lighting source. At night, with essentially no ambient light, the source of back lighting is energized and the cell is operated in a transmissive mode only. At other times, particularly during twilight, the LCD may be operated in both the reflective and the transmissive mode.